1. Field of the Invention
The present invention relates to a laser irradiation technique. For example, the invention relates to a technique of performing annealing or the like on a semiconductor film by irradiating it with a laser light.
2. Description of the Related Art
In recent years, techniques of crystallizing or improving the crystallinity of a non-single-crystal semiconductor film such as an amorphous semiconductor film or a crystalline semiconductor film (i.e., a non-single-crystal, crystalline semiconductor film such as a polycrystalline or microcrystalline semiconductor film) formed on an insulative substrate such as a glass substrate through laser annealing have been studied extensively. Silicon films are commonly used as such semiconductor films. The glass substrate is less expensive than and superior in workability to the quartz substrate that is widely used conventionally, and has an advantage that a large-area substrate can be produced easily. These are reasons for the above-mentioned studies. The reason why the laser is preferred for crystallization is that the glass substrate has a low melting point. The laser can supply high energy only to a non-single-crystal film without changing the temperature of a substrate much.
A crystalline silicon film formed by laser annealing has high mobility, and hence is widely used in, for instance, the monolithic liquid crystal electro-optical device in which thin-film transistors (TFTs) for pixel and TFTs for driver circuits are formed on a single glass substrate by using the crystalline silicon film. In general, a crystalline silicon film is called a polysilicon film because it is made up of a number of crystal grains.
Having high mass-productivity and being superior from the industrial viewpoint, a laser annealing method is preferred in which a pulse laser beam emitted from a high-power excimer laser or the like is shaped by an optical system into a several-centimete r-square spot or a line that is several millimeters wide and tens of centimeters long on the irradiation surface and the irradiation surface is scanned with the laser beam (the laser beam irradiation position is moved relatively to the irradiation surface).
In particular, the use of a linear laser beam provides high productivity because the entire irradiation surface can be irradiated with laser light by a scan only in the direction perpendicular to the longitudinal direction of the linear laser beam. This is in contrast to the case of using a spot-like laser beam which requires scans in two orthogonal directions. The reason why the scan is performed perpendicularly to the longitudinal direction of the linear laser beam is that this scanning direction is most efficient. Because of the high productivity, the use of a linear laser beam in laser annealing is now becoming the mainstream.
The technique of performing laser annealing on a non-single-crystal semiconductor film by scanning it with a pulse laser beam that has been modified into a linear shape has several problems. Among those problems, one of the particularly serious problems is that the laser annealing effect is not uniform over the entire film surface. At the stage when a linear laser beam started to be used, a phenomenon that stripes appeared at overlaps of beams was remarkable and a film exhibited marked differences in semiconductor characteristics between those stripes (see FIG. 1A).
For example, if a liquid crystal display is produced by using a film having such stripes, there occurs a problem that the stripes themselves are visible on the screen. This problem is now being solved by improving a non-single-crystal semiconductor film as a subject of laser irradiation and decreasing the pitch of a scan with linear laser beams (i.e., the interval between adjacent linear laser beams). An experiment of the inventors showed that a proper scanning pitch was about 1/10 of the width of a linear laser beam.
As the above-described striped pattern became less remarkable, the non-uniformity in the energy profile of a beam itself came to be more recognizable. In general, a linear laser beam is formed by passing an original rectangular beam through a proper lens group. The original rectangular beam having an aspect ratio of about 2 to 5 is deformed into a linear beam having an aspect ratio of 100 or more by, for instance, a lens group (called a beam homogenizer) shown in FIG. 2. This lens group is designed so as to uniformize also the intrabeam energy profile. The energy profile is uniformized by dividing the original rectangular beam, enlarging the respective divided beams, and then combining the divided beams together.
A simple consideration may lead to a conclusion that a beam obtained in the above manner by division and reconstruction would be higher in the degree of uniformity of the energy profile as the division is made finer. However, actually, in spite of fine division made on a rectangular beam, a striped pattern as shown in FIG. 1B appeared on a semiconductor film that was irradiated with such a beam. That is, innumerable stripes were formed perpendicularly to the longitudinal direction (that is, a width direction) of a linear laser beam. The formation of such a striped pattern should result from the lens group or the fact the original rectangular beam has a striped energy profile.
To investigate which of the two reasons caused the formation of a striped pattern, the inventors conducted a simple experiment. In the experiment, how vertical stripes vary was checked when an original rectangular leaser beam was rotated before entering the lens group. Vertical stripes did not vary at all. This made it clear that rather than the original rectangular beam the lens group is relates to the formation of a striped pattern. Since this lens group uniformize the energy profile of a single-wavelength beam that is equalized in phase (since the laser produces high-intensity light by equalizing the phase, resulting laser light is equalized in phase) by dividing and reconstructing it, an explanation is made that the stripes are interference fringes of light.
The light interference is a phenomenon that light beams having the same wavelength and phase intensity or weaken each other because of a deviation in phase when they are superimposed on each other with an optical path difference. FIG. 3 shows, in terms of light intensity I, interference fringes formed by five slits 301 that are arranged at regular intervals.
Where the five slits 301 are arranged at regular intervals, a peak of interference occurs at the center position A that corresponds to the center slit of them and interference fringes are formed with that peak as the center. If the diagram of FIG. 3 is applied to a lens system consisting of a cylindrical lens group 401 and a cylindrical lens 402 as shown in FIG. 4 (corresponding to the cylindrical lens group 203 and the cylindrical lens 205 in FIG. 4, respectively), one can see that the center point A of a linear beam in FIG. 4 corresponds to the center position A in FIG. 3 and a peak of interference appears at the center point A. The beam division number of the cylindrical lens group 401 in FIG. 4 corresponds to the number of slits 301 in FIG. 3. A cylindrical lens groups is also called a multi-cylindrical lens, a lenticular lens or a flyeye lens.
In each of FIGS. 3 and 4, as the position goes from the center point A to points B or C, the intensity of interference varies periodically. Actual interference fringes of a laser beam do not exhibit such a clear intensity variation. It is presumed that this is due to energy diffusion in a semiconductor film that is caused by heat conduction.
Incidentally, in FIG. 2, the combination of the cylindrical lens group 202 and the cylindrical lens 204 acts on a laser beam in the same manner as the combination of the cylindrical lens group 203 and the cylindrical lens 205. Therefore, it is understood that the same light interference occurs also in the width direction of a linear laser beam.
It is concluded from the above discussion that in FIG. 6 a linear laser beam 601 formed by the optical system as shown in FIG. 2 has a distribution of interference peaks 602 (indicated by circles) that are arranged in matrix form in the beam 601. This conclusion is easily derived by extending the light interference of FIG. 3 to a two-dimensional case. The intervals between interference peaks are not constant because the linear beam is composed of spherical waves (when a spherical wave is cut by a straight line, the intervals between points of the same phase are not constant).
The intervals between interference peaks can be made constant by composing a linear beam from plane waves (when a plane wave is cut by a straight line obliquely, the intervals between points of the same phase are constant). FIG. 5 shows an optical system for forming such a light wave.
The optical system of FIG. 5 is different from that of FIG. 4 in that laser beams divided by a beam-incidence-side cylindrical lens group 501 are converted into parallel beams by a downstream cylindrical lens 502. This type of optical system can easily be obtained by properly setting the distance between the upstream cylindrical lens group 401 and the downstream cylindrical lens 402 in FIG. 4.
In this manner, every beam divided by the cylindrical lens group 501 is modified into a plane wave by the cylindrical lens 502. Vertical stripes were given constant intervals by using beams that were formed by the optical system of FIG. 5.
As described above, a linear beam has a distribution of interference peaks that are arranged in matrix form. Therefore, if a scan is performed along the matrix while linear laser beams are overlapped with each other (the scanning direction is perpendicular to the longitudinal direction of the linear laser beams, that is the scanning direction is equal to the width direction of the linear beam), the same location of an irradiation object is repeatedly irradiated with beam portions that are high or low in intensity of interference. As a result, stripes due to strong or weak light are formed in the beam scanning direction.
The above-mentioned striped pattern is formed in such a manner that peaks of light interference that are distributed in the direction perpendicular to the longitudinal direction of a linear laser beam are emphasized by being superimposed on each other at a pitch that is sufficiently smaller than the beam width. FIG. 7 shows how stripes are formed in the direction perpendicular to the longitudinal direction of a linear laser beam. A linear laser beam 701 has a periodical energy variation in the longitudinal direction that is caused by light interference. (Although as described above the linear laser beam 701 has a periodical energy variation also in the width direction that is caused by light interference, this component does not influence the invention much.) Stripes emphasized if the linear laser beams 701 are overlapped with each other as shown in FIG. 7.
It was very effective to overlap linear laser beams with each other obliquely as shown in FIG. 8 so that a striped pattern is not emphasized.
This is because with this manner of irradiation peak portions of interference do not strike the same location many times and are distributed uniformly over the entire substrate. However, the processing method of FIG. 8 cannot utilize the full length of leaser beams.